Updated EVN station positions: FR005 reanalysis

FR005 was a 6-cm phase-reference test experiment. It comprised ~1hr of
interleaved observations of 3C345 & J1640+3946 (separation 0.5deg),
then ~1hr of 3C345 & J1635+3808 (separation 2.25deg), both using a
cycle time of 3.25min, and finally another ~1hr of 3C345 & J1640 at a shorter
cycle time of 2min. Seven EVN telescopes
participated: Ef, Mc, Nt, Wb_array, Jb2, On85, and Tr. Of these, only
Ef, Mc, and Nt participate regularly in geodetic campaigns and hence had
sub-cm positions in the original analysis.

FR005 was initially analyzed using the station positions in the SCHED
station catalogue. Problems with the phase residuals and phase-reference
maps were evident, and appeared consistent with station-position errors.
We used the unrelated Network Monitoring Experiment C00C1 (12 hours on DA913)
to estimate station-position adjustments based on residual delay-rates.
Delay-rate solutions of course
provide no information about the z-component of the station position, and
with only one source there was no possibility of treating atmospheric effects
in a satisfactory manner. Station-position offsets for Jb2, Wb_array, and
Tr were estimated from the C00C1 delay-rate residuals. A position of the
On85 antenna relative to the (geodetic) On60 antenna was provided by P.
Charlot and G. Elgered. A reanalysis of the FR005 observations was performed
using these updated positions, including the Wb axis offset that was missing
in the SCHED database. Improvements in the phase residuals and phase-reference
mapping were found, but there were still noticeable problems.

Patrick Charlot volunteered to conduct a 6cm test experiment using geodetic
techniques in order to determine optimal positions for EVN stations that do
not participate in geodetic campaigns (experiment TP001). He has now completed
analysis of this experiment sufficiently to obtain positions determinations
likely good to 10cm. We have done a further reanalysis of the FR005
observations with these TP001-derived positions, including the Jb2 axis
offset that he discovered to be also missing. The improvements over the
preliminary C00C1 reanalysis are evident, and are very significant compared
to the original situation.

The figure below shows phase
residuals on baselines from stations with updated positions to Ef for the
pair 3C345 - J1640+3946. Click on the
little image to download a full-size gzipped postscript file.
On85 did not participate during the first hour; the source-swapping rate was
too fast for Wb in the second. Three analyses were completed as above;
one using the original station positions, one using the C00C1 position
estimates, and one using the TP001 position determinations.
In each analysis, the two sources were FRINGed
separately, leading to a CL table "for" 3C345 and one "for" J1640.
The plot shows four sets of points for post-FRING phases of J1640 for each
baseline:

purple: calibrated using its own CL table. These residual
phases are ~0, and essentially the same regardless of the station-position
adjustments used.

grey-green: analysis using station positions as in the SCHED database;
calibrated using 3C345's CL table. Clear non-zero trends are seen.

light-blue: analysis using C00C1 station positions; calibrated using
3C345's CL table. The phase residuals approach
0, but there are still noticeable discrepancies, especially at Jb2.

The figure below corresponds to the previous one,
but for the pair 3C345 - J1635+3808; all analysis/plotting details are
the same. All stations participated. The slopes of the
uncorrected phase residuals are more severe here, since the pair has a
greater separation. The improvement in using the TP001 station-position
adjustments are similarly more evident, especially at Wb.

The pair 3C345 - J1635 also best illustrates the improvements in the
phase-reference maps themselves. We can compare the map of J1635+3808
made using its own CL table with those phase-referenced from 3C345
(CL tables applied cross-source during SPLIT).
All maps were made by IMAGR with the same adverb settings for
consistency in the comparisons (CELLSIZE=0.5mas, NITER=200, LOOPGAIN=0.1),
no self-cal in the "own-CL" map. Better maps could be produced with more
careful tinkering with the CLEAN-control parameters. The below table
summarizes the features
of the J1635 maps for each of the four cases:

Map Type

Peak FD

Total FD

SNR

[Jy/bm]

[Jy]

own CL table

2.15

2.42

320

PhRef, orig pos.

0.49

1.88

12

PhRef, C00C1 pos.

1.11

1.96

39

PhRef, TP001 pos.

1.55

1.99

62

The figure below shows the maps of J1635+3808 for the four cases described
in the table. The images were plotted with the same color map in each case,
ranging from -0.1 to 0.9 Jy/bm. Click on the image
to download the full-size gzipped postscript file.
The map of J1635 made with its own CL table is in the upper left.
This is followed, left to right and top to bottom, by the phase-reference
maps using the original, C00C1, and TP001 station positions/axis offsets of
On85, Tr, Wb, and Jb2. The problems with the phase-reference map
using the original positions are evident, as is the improvement
in focusing the flux into the source and the reduction of the off-source
rms when the TP001-derived positions are used.

The phase-referencing is especially
sensitive to the station-position improvements because 4 of the 7 stations
initially had station positions that were 1-5 meters off. See the
recipe for incorporating the TP001
station-position adjustments into already-correlated data for a more
quantitative summary of the position adjustments.

In January 2002, the calculation of the offset between the
single-dish and tied-array Wb positions was refined. Improvements were
checked in the Network Monitoring experiment N01C3, which performed
phase-referencing between DA193 and J0605+4030, 2.09deg apart,
at 5GHz. The removal of a ~150deg drift in Ef-Wb, Mc-Wb, and
Jb-Wb was evident. The Wb array position in the
recipe includes this refined correction.